It is of paramount importance to the operation of interferometers, and consequently to the performance of spectrometers incorporating them, that accurate information be constantly available as to the precise position of the moving interferometer reflectors(s). Direction, velocity, and location data are necessary for control purposes as well as for correlation of reflector position with the analysis beam detector signal, which is of course essential for the generation of appropriate and accurate frequency domain information.
Tracking of mirror position is normally carried out by use of a monochromatic radiation beam (usually from a laser), which enters the interferometer in parallel with the analytical spectrometer radiation beam and produces a sinusoidal output signal from the photoconductive detector upon which it is directed. A zero crossing of the sine wave occurs with each quarter-wave-length movement of the reflector, which crossings are then counted and used for the determination of reflector position.
The so-called “quadrature” tracking technique employs two parallel monochromatic radiation beams, out of phase (usually by 90°) with one another, to provide four pulse trains to more accurately indicate mirror position. Apparatus employing that technique is described, for example, in Badeau U.S. Pat. No. 4,847,878.